1. Field of the Invention
The present invention relates to a semiconductor laser in which at least one end of a resonator is equipped with a window structure and a method of making the same.
2. Related Background Art
There have been known semiconductor lasers in which, in order to attain higher output, a window structure transparent to laser light is provided near a light exit end of a resonator having a quantum well active layer, thereby preventing catastrophic optical damage (COD) at this light exit end facet.
For example, in the semiconductor laser disclosed in Japanese Patent Application Laid-Open No. 7-58402, impurities are introduced in the vicinity of a resonator facet of a Fabry-Perot type semiconductor laser having a quantum well structure, thereby forming a window structure. Namely, GaInP or AlGaInP crystal material is grown under a predetermined condition, thus forming an active layer with spontaneous ordering structure, and then impurities such as Zn are introduced into a light exit end region of the resonator having this active layer, thereby forming a window structure in which the spontaneous ordering structure is disordered. Since the bandgap of thus obtained window structure region becomes greater than that in a region which is not disordered, catastrophic optical damage (COD), which tends to occur during a high output operation of a semiconductor laser, is suppressed, thereby higher output is realized.
In the semiconductor laser having a window structure disclosed in U.S. Pat. No. 4,845,725, injected impurities (Si, Zn, and Sn) are diffused to an multiple-quantum well (MQW) active layer, so as to cause interdiffusion between well layers and barrier layers, thereby realizing a window structure having a greater bandgap in this part. Thus obtained window structure also functions as a transparent optical waveguide layer in which the guided mode of light gradually broadens, thereby minimizing the size of a far-field laser light image.
The semiconductor laser disclosed in Japanese Pat. Publication No. 6-48742 is an AlGaAs type semiconductor laser in which Zn is selectively diffused into an AlAs/GaAs superlattice active layer, thereby the superlattice structure is totally disordered. The bandgap of this disordered region becomes greater than that of the active layer without Zn-diffusion and the disordered region can be a window strcture.
Having studied the conventional semiconductor lasers, the inventors have found the following problems. Namely, known as the conventional semiconductor lasers are those of a type in which impurities are introduced so as to disorder a spontaneous ordering structure, thereby forming a window structure (Japanese Pat. Application Laid-Open No. 7-58402); and those of a type in which impurities are introduced so as to cause diffusion between the well layers and barrier layers in a quantum well layers, thereby forming a window structure (U.S. Pat. No. 4,845,725 and Japanese Pat. Publication No. 6-48742). In these semiconductor lasers, however, since impurities introduced in order to form a window structure exist in the window structure, light is absorbed by these impurities, and thereby the performance of the semiconductor lasers may be deteriorated. Also, due to such impurities, leak current tends to increase in the vicinity of the laser facet. Such leak current causes nonradiative recombination of carriers, and the energy caused by the nonradiative recombination accelerates chemical reactions such as oxidation. Consequently, the laser facets may be deteriorated, and COD may be occurred at the deteriorated facets. In the case where impurities are introduced by means of ion implantation, crystals may be damaged by the implantation, which causes reliability of the laser to deteriorate.